WASHINGTON — President Trump’s declaration that “we will win” in Afghanistan came with bellicose language about taking “fast and powerful” retribution against militants and a promise to succeed where other presidents have failed to resolve the nearly 16-year-old conflict.

Left unclear by the Trump administration was the answer to a question that has stumped Washington, including the two previous commanders in chief, since the United States became involved in the war there: What does winning look like in Afghanistan?

For Mr. Trump, winning looks a lot like a very long war.

“From now on, victory will have a clear definition,” Mr. Trump said on Monday during his prime-time televised address on his strategy for Afghanistan. “Attacking our enemies, obliterating ISIS, crushing Al Qaeda, preventing the Taliban from taking over Afghanistan, and stopping mass terror attacks against America before they emerge.”

But hours before Mr. Trump spoke, the president’s top advisers offered a far different view of success, arguing that the ultimate solution to the conflict in Afghanistan was a settlement between the Afghan government and the Taliban, who would theoretically be beaten down by the Americans and forced to the negotiating table.

Mr. Tillerson endorsed that approach on Tuesday, arguing that the president had laid out a strategy “intended to put pressure on the Taliban.”

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Before he became a candidate for president, Donald J. Trump was clear about Afghanistan. But during his campaign, he switched views.Published OnAug. 21, 2017CreditImage by Stephen Crowley/The New York Times

The idea, Mr. Tillerson said during a rare appearance in the State Department briefing room, was “to have the Taliban understand, you will not win a battlefield victory — we may not win one, but neither will you — so at some point, we have to come to the negotiating table and find a way to bring this to an end.”

Among military commanders and others who have studied the war in Afghanistan, the definition of success is even broader: winning back territory from the Taliban, building up the Afghan armed forces and rooting out endemic corruption in the government there. Mr. Trump mentioned all in his address.

The multiple definitions of success are hardly new, and were in fact the central ideas behind the surge of forces in Afghanistan that the Obama administration launched in 2010. None of the ideas worked, even when there were 100,000 American troops in Afghanistan. The number of American troops in the country now stands at 8,400, and Trump administration officials say the president is likely to add as many as 4,000 more.

The Taliban in the meantime have continued to gain territory. Insurgents overran a remote district in northern Afghanistan on Monday, the sixth government-held area to fall to the Taliban in the past month.

Secretary of State Rex W. Tillerson endorsed Mr. Trump’s plan during a rare news conference at the State Department on Tuesday.CreditBrendan Smialowski/Agence France-Presse — Getty Images

Neither the president nor his aides have specified the conditions that would have to be met for American forces to leave. But officials have said Mr. Trump would recognize winning in Afghanistan when he saw it, and that in turn would determine the extent of the United States commitment and staying power in Afghanistan.

“Essentially, without committing to troop levels or timelines, President Trump has said our strategy is to outlast the Taliban and other terrorist groups, and to only leave Afghanistan once the Afghan government and its armed forces can maintain control of the country,” said Senator Chris Coons, Democrat of Delaware, a member of the Foreign Relations Committee.

Most of the coalition troops are focused on training and advising the Afghan military, which has struggled with corruption and morale problems and whose soldiers are now being killed at an alarming rate (31 a day, according to official data). A smaller number of Americans, most of them special forces, are primarily hunting the remnants of Al Qaeda and a branch of the Islamic State that has made inroads in recent years.

Jim Mattis, the secretary of defense, said Tuesday that he had yet to determine the number of additional troops to send to Afghanistan, but deployments are expected quickly. Gen. Joseph L. Votel, the head of United States Central Command, said Tuesday that additional trainers, advisers and Special Operations troops would start arriving in Afghanistan in the next “days or weeks” to take advantage of the last two months of the fighting season.

For President George W. Bush, who sent American troops to Afghanistan in 2001, victory was supposed to be liberating the Afghans from the Taliban and ensuring that the country could not be used as a base for terrorists to launch attacks against the United States. But he quickly realized that achieving that goal would mean nation-building — an endeavor explicitly rejected by Mr. Trump — because Afghanistan had no functioning state and had to be rebuilt.

Lt. Gen. H. R. McMaster, Mr. Trump’s national security adviser, played a central role in President Barack Obama’s effort in Afghanistan as the leader of one of the American task forces that sought to root out the rampant corruption that had taken hold in the Afghan government. During his time in Kabul, General McMaster would frequently make the point that “criminal patronage networks” — that is, predatory government officials working together to enrich themselves — were one of the two greatest threats to Afghanistan (Taliban sanctuaries in Pakistan being the other).

Yet dismantling the networks proved far easier said than done. The corruption went all the way to the top of the government, and was tacitly encouraged by some parts of the United States government, even as others parts tried to combat it.

As a result, neither General McMaster’s task force nor other American anti-corruption efforts enjoyed any meaningful success, and the problem has only intensified. It has now spread deep into the ranks of the Afghan military, whose leaders have enriched themselves, leaving soldiers exposed and demoralized on the battlefield.

“The only way Afghanistan will stabilize is if it has a government the Afghan people want,” said John Dempsey, a former senior State Department official who worked on Afghanistan and Pakistan.

John Nagl, a retired Army officer and counterinsurgency expert who fought in both wars in Iraq, said that in turning away from his impulse to withdraw American troops from Afghanistan, Mr. Trump was in essence committing to a lengthy and costly effort to stabilize the country, rein in the Taliban and other militants and ensure that Pakistan’s nuclear weapons did not fall into the hands of terrorists — goals that could take a generation to achieve.

“It’s going to take a long time — we’re halfway there — and it’s not a very satisfying end state,” Mr. Nagl said, noting that success would probably involve American casualties and maintaining United States military bases in Afghanistan for decades to come. “We should have learned by now that wars have long tails and unexpected consequences.”